Most conventional DC-DC and AC-DC converters have a transformer with primary and secondary switching elements, wherein the isolation from the primary winding and secondary winding creates the separation between primary and secondary. To improve the efficiency of such converters, engineers looked for solutions to obtain zero voltage switching across the primary switches at “turn on” and to ensure that the current through the rectifier means connected to the windings, reach zero before the rectifier means turn off. In this context, rectifier generally identifies electronic devices which are designed to conduct current in a unidirectional way; those devices can be diodes, or controlled MOSFETS which do emulate the function of a diode, referred to in the power conversion field as synchronous rectifiers.
Another practices includes “true soft switching”. In a “true soft switching” converter the primary switching elements turn on at zero voltage switching conditions and the rectifier means turn off at zero current. In a “true soft switching” converter, there is no ringing or spiking across any of the switching elements.
In the last 30 years, there have been many solutions developed to ensure zero voltage switching across the primary switches. Many such solutions require additional components and in most cases still lead to an increase in conduction losses. This includes full bridge topologies used in higher power applications which do obtain zero voltage switching for the switching element, such as U.S. Pat. Nos. 5,231,563 and 6,862,195, 7,009,850.
All these technologies do offer zero voltage switching under certain conditions across the primary switching elements but they do not always create zero current at turn off for the rectifier means.
Resonant topologies that became very popular in the last ten years shape the current through the switching elements in a half sinusoidal shape in order to create conditions for zero voltage switching in primary and sometime zero current switching through the secondary rectifier means. Shaping the current from rectangular shape to half sinusoidal shape does increase the root mean square (“RMS”) current and as a result increases the conduction losses. In addition, in resonant topologies, the modulation of output power is done through modulation in frequency which in some applications is not acceptable.
In conventional constant frequency pulse width modulation (“PWM”) topologies, there are solutions to obtain zero voltage switching in the primary and even zero current switching through the rectifier means (such as presented in U.S. Pat. No. 10,103,639). However, in these topologies, there were some penalties such as an increase in conduction losses and some restriction of the input voltage range and output current range.
In addition, many of these solutions apply to specific topologies. Such is the case in U.S. Pat. No. 9,985,546 for full bridge topologies, U.S. Pat. No. 9,899,928 for half bridge and full bridge topologies, and U.S. Pat. No. 7,450,402 for flyback topology.